Conditions were characterized for maximizing the uptake of exogenous mammalian cell DNA by enzyme-deficient Chinese hamster lung cells. These optimal conditions will be applied to the SV40 transfection system. It was determined that a correlation exists between the persistence of intact virus and the presence of infectious DNA-protein complexes in SA7-induced tumor cells. The nature of the infectious moiety in the SA7 tumor cells is being characterized. Bibliographic references: Brugge, J.S. and Butel, J.S.: Role of simian virus 40 gene A function in maintenance of transformation. J. Virology 15: 619-635 (1975). Farber, F.E., Melnick, J.L. and Butel, J.S.: Optimal conditions for uptake of exogenous DNA Chinese hamster lung cells deficient in hypo-xanthine-guanine phosphoribosyltransferase. Biochem. Biophys. Acta 390: 298-311 (1975).